The present invention relates to a method for quantitatively analyzing metals solid-solubilized in a silicon wafer.
It is known that, with recent use of finer semiconductor devices and higher integration degree thereof, metals contained in silicon wafers degrade device characteristics and markedly influence on yield of device production. In particular, it is known from many examples that Cu solid-solubilized in silicon wafers (also referred to as xe2x80x9cbulk Cuxe2x80x9d hereinafter) may be a cause of bad influences on the device characteristics. Therefore, a large number of gettering methods, cleaning methods and so forth have been researched for eliminating such metal impurities.
On the other hand, there is desired a method for analyzing such metal impurities, especially bulk Cu concentration, with high precision and high sensitivity for controlling the metal contamination during the wafer production processes such as polishing step and cleaning step.
As such a method for analyzing metals contained in silicon wafers, there are evaluation methods called one-drop method, step etching method and so forth. These are dissolution methods in which all or a part of a silicon wafer is dissolved with a mixture of hydrofluoric acid and nitric acid (also referred to as xe2x80x9cmixed acidxe2x80x9d hereinafter) or the like in a gaseous phase or liquid phase and metals in the solution are quantified by an analysis apparatus.
There is also a method called an annealing combined method or the like. In this method, metals present in a silicon wafer is transferred to a wafer surface (or captured thereby) by subjecting the wafer to a heat treatment, then an oxide film on the wafer surface is decomposed in a vapor phase, recovery solution is run over the entire surface of the wafer, and the recovery solution is subjected to quantitative analysis using an analysis apparatus.
Analysis apparatuses generally used for the analysis of dissolution solution or recovery solution obtained by these methods include frameless atomic absorption spectrophotometer (abbreviated as xe2x80x9cAASxe2x80x9d hereinafter), inductively coupled plasma mass spectrometer (abbreviated as xe2x80x9cICP-MSxe2x80x9d hereafter) and so forth.
Further, although it is not a method of directly evaluating metals contained in silicon wafers, there are also contemplated a method of directly analyzing metals contained in chemical solutions used in cleaning step and so forth, and other methods.
The conventional dissolution methods require enormous labor for maintenance and management of analysis apparatus and prevention of contamination from environment, and are likely to suffer from significant fluctuation of human factors. For example, when Cu is contained in a silicon wafer in an amount of about 1xc3x971013 atoms/cm3 and the analysis is performed by the dissolution methods, detection cannot be achieved unless the analysis apparatus used for analyzing a final dissolution solution has an ability to detect about 0.1 ppt of the metal.
Further, for example, analysis may become possible at a level exceeding the ability of the analysis apparatus by concentrating the dissolution solution and so forth. In such a case, however, contamination which is newly introduced from a platinum crucible used for the concentration or external environment, i.e., interfusion of metals, is expected, and therefore good measurement precision could not necessarily be obtained.
In recent years, the sensitivity of analysis apparatuses, such as frameless atomic absorption spectrophotometer (AAS) and inductively coupled plasma mass spectrometer (ICP-MS), has shifted to sensitivity of ppt level. As for the ability of generally used apparatuses such as frameless atomic absorption spectrophotometer and inductively coupled plasma mass spectrometer, for example, the frameless atomic absorption spectrophotometer is at about 100 ppt level, and even the inductively coupled plasma mass spectrometer is at about 1 ppt level for Cu.
However, for evaluation of metals in silicon bulk, it is a problem how to extract metals contained in inside of silicon to the surface and collect them, in addition to the increase of sensitivity of the analysis apparatus itself.
When Cu is analyzed by the dissolution methods such as the one-drop method and the step etching method by using currently used apparatuses, the analysis could not actually be performed unless 1xc3x971014 to 1xc3x971016 atoms/cm3 or more of Cu is contained in a silicon wafer, because the apparatuses show bad recovery yield from inside of the silicon.
Furthermore, in the annealing combined method, although metals comes to be likely to gather at the surface by the heat treatment, the recovery yield, i.e., the ratio (gettering efficiency) of metals transferred to the wafer surface (captured at the wafer surface) is as low as 0.1% or less for Cu in low resistivity silicon wafers which are doped with boron at a high concentration and so forth. Therefore, metals contained in the inside of silicon scarcely gather at the surface, and also there is resistivity dependency, so that measurement has a large error. In addition, it is expected that contamination and so forth are newly invited by the heat treatment at a high temperature (about 650xc2x0 C.). Even by this method, the analysis could not actually be performed unless 1xc3x971013 to 1xc3x971014 atoms/cm3 or more is contained in a silicon wafer.
Further, the method of directly analyzing metals contained in a chemical solution used in cleaning step etc., for example, suffers from problems that detection requires concentration of the solution by heating or the like because only a small amount of metal impurities are contained in a large volume of the chemical solution, the chemical solution may be an inhibition factor for the analysis depending on its nature, and strikingly decrease sensitivity of the analysis for metals when a large volume of the chemical solution is present. For example, these are caused in a case where Cu should be analyzed in a cleaning solution containing a large volume of sulfuric acid. For the analysis of Cu contained in such a solution, a special method such as an evaluation method utilizing a radioisotope must be used, and thus the method must be in a large scale in view of evaluation time and apparatus. Further, this is just an evaluation of metals in the chemical solution, and it is not for accurately determining concentration of metal impurities in a silicon wafer, which is the original matter of interest.
Accordingly, an object of the present invention is to provide a pretreatment method for analyzing a concentration of a metal, in particular, Cu, contained in a silicon wafer with high sensitivity in a simple manner.
In order to achieve the aforementioned object, the present invention provides a method for evaluating concentration of metal impurities contained in a silicon wafer, which comprises dropping concentrated sulfuric acid onto a surface of the silicon wafer to extract metal impurities solid-solubilized in the inside of silicon wafer into the concentrated sulfuric acid, and chemically analyzing metal impurities contained in the concentrated sulfuric acid.
If the metals in the bulk are recovered by using concentrated sulfuric acid as described above, the metals once recovered in the concentrated sulfuric acid scarcely diffuse again into the inside of the bulk, and thus the metals can be efficiently extracted to the wafer surface. Further, the wafer surface is unlikely to be roughened, and therefore favorable evaluation of wafer can be performed. Moreover, since the evaluation can be performed by using a few drops of concentrated sulfuric acid, the influences of sulfuric acid such as decrease of analytical sensitivity can be minimized.
Specifically, the method for extracting metal impurities solid-solubilized in the inside of a silicon wafer into concentrated sulfuric acid is performed by dropping an arbitrary amount of concentrated sulfuric acid onto the aforementioned silicon wafer surface, putting another uncontaminated wafer on the concentrated sulfuric acid on the aforementioned silicon wafer to hold the concentrated sulfuric acid between the wafers, and subjecting the whole of the wafers in that state to a heat treatment.
The uncontaminated wafer is put on the dropped concentrated sulfuric acid in order to facilitate the sulfuric acid to uniformly spread over the entire surface of the wafer. Further, it is also used in order to prevent the concentrated sulfuric acid from rapidly evaporating or scattering during the heat treatment to secure safety.
Therefore, the material of the uncontaminated wafer used for preventing scattering of concentrated sulfuric acid and so forth (also referred to as a xe2x80x9cprotective waferxe2x80x9d) is not particularly limited, and it is also possible to use quartz glass or the like. However, if spread of the concentrated sulfuric acid is taken into consideration, use of a silicon wafer, in particular, such a wafer having a surface subjected to etching treatment (also referred to as xe2x80x9cCW waferxe2x80x9d) provides favorable uniform spread of concentrated sulfuric acid over the entire surface of wafer, and easy delamination after the treatment. Moreover, if the CW wafer is subjected to a treatment with concentrated sulfuric acid beforehand, metal impurities in the CW wafer can be eliminated. By using such a CW wafer, contamination of a wafer to be evaluated from the CW wafer (or contamination of the concentrated sulfuric acid from the CW wafer) can be suppressed as much as possible, and thus the precision of the evaluation can be increased. Further, a wafer of n-type is originally unlikely to be contaminated, and therefore particularly suitable for the protective wafer.
The aforementioned heat treatment is preferably performed at a temperature in the range of 100xc2x0 C. to 290xc2x0 C.
This is because diffusion of metal impurities is promoted so that transfer of the impurities from inside of the wafer to the wafer surface should be facilitated.
As for the method of chemically analyzing metal impurities contained in the concentrated sulfuric acid, after metal impurities solid-solubilized in the inside of silicon wafer are extracted into the concentrated sulfuric acid, the concentrated sulfuric acid on the silicon wafer is neutralized by exposing it to an ammonia gas atmosphere, and then a recovery solution for recovering metals remaining on the silicon wafer is dropped onto the wafer surface, run over the wafer surface, then recovered and chemically analyzed.
This procedure enables efficient recovery of metals gathered on the wafer surface.
The aforementioned recovery solution consists of hydrofluoric acid/aqueous hydrogen peroxide, hydrochloric acid/aqueous hydrogen peroxide, hydrofluoric acid/nitric acid or aqua regia. Since these recovery solutions are coexistent with an oxidizing agent, they enable easy recovery of metal impurities.
Further, the aforementioned chemical analysis may be frameless atomic absorption spectrometry or inductively coupled plasma mass spectrometry.
This is because apparatuses used for these analyses are generally used apparatuses and they can analyze chemicals used for the present invention and so forth. However, the chemical analysis method is not limited to these, and the apparatus may be an apparatus that can analyze a solution recovered according to the present invention with further higher sensitivity.
Among evaluations of metals contained in silicon wafers, the analysis method of the present invention is particularly preferred for analysis of Cu. While metal impurities such as Cu, Ni, Ag and the like are basically recovered in concentrated sulfuric acid, Cu, in particular, is likely to remain in the bulk, and it is the current major problem that it cannot be recovered efficiently. The present invention concerns evaluation in which such Cu can be recovered at high yield.
Moreover, the method of the present invention is particularly suitable for evaluation of a silicon wafer having a resistivity of 1 xcexa9xc2x7cm or less.
A wafer showing low resistivity of 1 xcexa9xc2x7cm or less is significantly influenced by the resistivity in the conventional annealing combined method and so forth, and Cu and so forth can scarcely be recovered when the resistivity is low (amount of dopant is large). That is, it has been particularly difficult so far to evaluate metal contamination in the bulk of low resistivity wafer.
However, the method of the present invention suffers from little influence of resistivity, and it enables evaluation for a wide range of resistivity covering from a high resistivity wafer of 10 xcexa9xc2x7cm or higher to a low resistivity wafer of about 0.001 xcexa9xc2x7cm.
As clearly understood from the above explanation, the recovery yield of metals from inside of wafer is increased by the treatment with concentrated sulfuric acid according to the present invention. Further, it enables quantitative evaluation of a low resistivity wafer, which cannot have been evaluated thus far. Even by using a currently used apparatus for frameless atomic absorption spectrometry (lower detection limit is about 100 ppt), the analysis is possible and the sensitivity is improved, if about 1xc3x971011 atoms/cm3 of impurities are contained in a silicon wafer.
Further, according to the present invention, fluctuation of precision due to exogenous factors is also suppressed, and the method enables detection even at a low Cu concentration, if the Cu is contained at a concentration of 1xc3x97109xe2x88x9211 atoms/cm3 or more, and sensitivity and measurement precision are also improved.
FIG. 1 is a schematic flow diagram representing the evaluation procedure according to the present invention.
FIG. 2 is a schematic view of an exemplary heat treatment apparatus used for the present invention. In the figure, (a) is a plan view and (b) is a side view.
FIG. 3 shows relationship between Cu concentration in recovered solution and Cu concentration contained in inside of wafer. In the figure, A represents the relationship in the method of the present invention, B represents the same in the annealing combined method, C represents the same in the one-drop method, and D represents the same in the step etching method.